Ring laser angular rate sensors are well known and are particularly described in U.S. Pat. No. 3,373,650, issued to Killpatrick, U.S. Pat. No. 3,390,606, issued to Podgorski, and U.S. Pat. No. 4,151,071, issued to Podgorski, all of which are assigned to the assignee of the present invention. The above-referred to patents are incorporated herein by reference thereto. Ring laser angular rate sensors of the type referred to commonly employ a block of material that is dimensionally stable, both thermally and mechanically. The block usually includes a plurality of interconnected gas containing tunnels or passages which form a closed-loop path in the shape of a triangle, a rectangle, or any polygonal path. At each intersection of a pair of interconnected tunnels is a mirror mounted on the block. This arrangement of mirrors and interconnected tunnels forms an optical closed-loop path or cavity. Further, at least one anode and one cathode are each mounted on the block and in communication with the gas. Each of the components, including the mirrors, anode, and cathode, must be sealed to the block to form a gas tight seal. The block is usually filled with a lasing gas such as a mixture of helium and neon. A sufficiently large electrical potential is applied between the anode and cathode to cause a discharge current therebetween which results in the production of a pair of counter-propagating laser beams within the block.
Associated with ring laser angular rate sensors is a source of error usually referred to as "lock-in." The source of error is thought to be predominantly caused by back scattering of light at each of the mirrors which form in part the optical closed-loop path which the counter-propagating laser beams traverse. As is well understood by those skilled in the art, there are two widely used techniques applied together to minimize the lock-in error. The first technique consists of dithering the block as taught in U.S. Pat. No. 3,373,650. Mechanically dithering the laser block reduces the source of error caused by lock-in to acceptable levels such that ring laser angular rate sensors have become commercially successful. The second technique consists of producing mirror assemblies structured so as to provide highly polished substrates having superior reflective coatings which achieve minimal laser beam scattering at the surfaces thereof. Development of the mirror assemblies over the years has made it possible for high performance ring laser angular rate sensors.
U.S. Pat. No. 3,697,181, issued to Macek, et al teaches another technique for eliminating the lock-in effect in ring lasers. In the apparatus of Macek, a portion of each of the counter-propagating laser beams is extracted from the cavity and reflected back into the cavity in the direction of the oppositely traversing beam. The redirected beam is controlled in phase and amplitude in a manner as described in the specification of the aforementioned patent in order to reduce the effects of lock-in.